check: track a set of checked packs for partial checks

[mr-raj12]

The repository check writes a marker holding the last pack id it verified, so a partial check (--max-duration) resumes by skipping every pack up to that marker. This assumes the pack listing is sorted.

Pack files are named by the sha256 of their content, so a pack added between two partial runs can sort anywhere. If its id sorts before the marker, the resumed check skips it and never verifies it.

This replaces the marker with a persisted set of checked pack ids, a HashTableNT mapping pack id to (timestamp, result). A pack is skipped only when it appears in the set recorded as intact; a pack recorded as corrupt is re-verified rather than skipped. So a newly added pack gets checked whatever its name sorts to, and a pack that was corrupt in an earlier run is looked at again. The set is dropped once a cycle has checked every pack.

The set is managed by a PackTracker class. It persists via store.store() (atomic write) with a sha256 appended over the serialized table; on load a blob whose sha256 does not match is discarded, so a rotted set re-checks everything instead of skipping an unverified pack. Ctrl-C is therefore safe: the on-disk set is never partially written and never records a pack that was not verified. The cycle is checkpointed every minute, so an interrupt redoes at most a minute of verification work.

Closes #9897

[codecov]

Codecov Report

❌ Patch coverage is 79.54545% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.32%. Comparing base (7da670c) to head (b9214b6).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/repository.py	79.54%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9909      +/-   ##
==========================================
+ Coverage   85.13%   85.32%   +0.19%     
==========================================
  Files          93       93              
  Lines       15899    15926      +27     
  Branches     2428     2429       +1     
==========================================
+ Hits        13535    13589      +54     
+ Misses       1654     1633      -21     
+ Partials      710      704       -6     

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[ThomasWaldmann]

Also: does it handle Ctrl-C correctly?

[mr-raj12]

Also: does it handle Ctrl-C correctly?

Yes, store.store() writes atomically and appends a sha256 that's verified on load, so an interrupt can't corrupt the set or mark a pack checked when it wasn't. The worst case you redo a last one minute of work, capped at 1 min now that checkpoints run every 60s.